Silicon nitride sinter having high thermal conductivity and process for preparing the same

ABSTRACT

A high thermal conductive silicon nitride base sintered body which comprises a phase comprising crystal grains of silicon nitride and a grain boundary phase containing a compound of at least one element selected from the group consisting of yttrium and the lanthanide elements in an amount of 1 to 20% by weight in terms of oxide amount, and contains free silicon dispersed therein in an amount of 0.01 to 10% by weight based on the whole. This high thermal conductive silicon nitride base sintered body has high strength coupled with high thermal conductivity and thus is useful not only as various parts for semiconductor devices, such as radiating insulating substrates, but as various structural parts for machines, OA apparatuses, etc.

TECHNICAL FIELD

This invention relates to an Si₃ N₄ base sintered body which is usefulnot only as various parts for use in semiconductor devices, includinginsulating substrates and various radiating plates, but also as variousstructural parts for motor vehicles, machines, OA apparatuses, etc. andis excellent in productivity and especially in mechanical strength andradiating properties. This invention also relates to a process forproducing the sintered body.

BACKGROUND ART

Ceramics comprising silicon nitride as the main component are superiorin heat resistance, mechanical strength, and toughness to other ceramicmaterials, and are materials suitable for various structural parts suchas automotive parts and OA apparatus parts. Attempts are being made touse them as insulating radiating substrates for semiconductor devices,etc. so as to take advantage of their high insulating properties.

Alumina and the like have conventionally been used extensively asceramic substrates for semiconductors. However, with the trend towardhigher speeds, higher degrees of integration, and higher outputs insemiconductor devices, materials having higher thermal conductivity andexcellent radiating properties have come to be desired and theapplication of AlN and SiC has progressed. However, no high thermalconductive substrate has been obtained so far which is made of AlN orthe like and is sufficient in strength and toughness, and the currentsubstrates have drawbacks in product handling and shape because ofbreakage caused by external force, etc. There is hence a desire for thedevelopment of a ceramic material combining high-strength propertieswhich enable the material to withstand external force with excellentradiating properties.

Silicon nitride (Si₃ N₄), which intrinsically has high strength, isexpected to be used as insulating radiating substrates if its thermalconductivity can be improved. However, since the conventionally knownsilicon nitride sintered bodies have lower thermal conductivities thanAlN and SiC, they have not been put to practical use as an insulatingradiating substrate.

The thermal conductivity of insulating ceramics such as silicon nitrideis mainly attributable to the transmission of phonons. Since phonons arescattered by phases having different impedances, such as lattice defectsand impurities, present in the sintered body, the thermal conductivity κis defined by the following numerical formula 1:

    κ=c×V×l/3                                (Numerical formula 1)

(wherein c is specific heat capacity; V is average velocity of phonons;and l is the mean free path of phonons).

The specific heat capacity c and the group velocity V in numericalformula 1 each is a number which varies from material to material andcan be regarded as almost the same in the same material. Consequently,the thermal conductivity of silicon nitride crystal grains is governedsubstantially by the mean free path of phonons. For example, when AlN orAl₂ O₃, which have conventionally been used generally, is added as asintering aid, then aluminum ions or oxygen ions form a solid solutionin Si₃ N₄ crystal grains and thus scatter phonons, resulting in areduced thermal conductivity. Because of this, general silicon nitridebase sintered bodies to which Al₂ O₃, AlN, Y₂ O₃, or the like has beenadded have a thermal conductivity as low as about 15 W/m·k.

Various investigations have hence been made in order to obtain a siliconnitride base sintered body having a high thermal conductivity. Forexample, the thermal conductivity of a silicon nitride base sinteredbody obtained through an HIP treatment after the addition of Y₂ O₃ andAl₂ O₃ in combination as a sintering aid is discussed in "Paper Journalof Ceramics Society of Japan)," Vol.97 (1989), No.1, pp.56-62. Theresult given therein is that the thermal conductivity of the sinteredbody becomes higher as the proportion of β-form crystal grains increasesor as the proportions of Y₂ O₃ and Al₂ O₃ in the sintering aid increasesand decreases, respectively. There is a description in the paper,section 4.2 to the effect that high thermal conductivity is obtained byβ-form crystal grains because β-form crystal grains have a larger meanfree path of phonons than α-form crystal grains.

It is therefore important for heightening the thermal conductivity of asilicon nitride base ceramic to accelerate the formation of β-form Si₃N₄ crystal grains, to use a rare earth element compound such as Y₂ O₃,which is regarded as less apt to form a solid solution in the crystalgrains, and to diminish the addition of an aluminum compound containingaluminum ions, which are apt to form a solid solution in the crystalgrains.

For example, Japanese Patent Laid-Open Nos. 175268/1992 and 219371/1992show a case in which a dense silicon nitride base sintered body having athermal conductivity of 40 W/m·k or higher and consisting of β-form Si₃N₄ crystal grains was obtained by using a β-form Si₃ N₄ powder reducedin the contents of oxygen and cationic impurities so as to diminish theamounts of cationic impurities such as aluminum and oxygen, which form asolid solution in Si₃ N₄ crystal grains, and by additionally adding acompound of, e.g., a Group 4A element when a colored sintered body wasto be obtained.

Japanese Patent Laid-Open No. 30866/1997 discloses a dense siliconnitride base sintered body having a thermal conductivity of 80 W/m·k orhigher and a flexural strength of 600 MPa or higher which is obtained byadding a compound of an alkaline earth/rare earth element and conductingsintering in high-pressure nitrogen gas at a relatively high temperaturearound 2,000° C. to thereby heighten the proportion of large β-formcrystal grains having a minor diameter of 5 μm or larger.

Japanese Patent Laid-Open Nos. 135771/1994 and 48174/1995 disclose amethod for obtaining a dense silicon nitride base sintered bodyconsisting of β-form crystal grains which comprises adding anappropriate amount of aluminum ions together with a rare earth elementcompound and a Group 4A element compound, without limiting the amount ofaluminum ions, and gradually cooling the shape after sintering tothereby accelerate crystallization in the grain boundary phase. There isa description in these patent documents to the effect that an Si₃ N₄base sintered body having a flexural strength of 800 MPa or higher and athermal conductivity of 60 W/m·k or higher is obtained.

On the other hand, Japanese Patent Laid-Open Nos. 149588/1995,319187/1996, and 64235/1997 disclose: a metallized substrate comprisinga silicon nitride base and formed thereon a high-melting metallizinglayer made of tungsten or molybdenum; and a semiconductor modulecomprising the substrate and a conductor circuit bonded thereto.Japanese Patent Laid-Open No. 187793/1995 discloses varioussemiconductor devices containing a similar metallized substrate andvarious structural members comprising the same silicon nitride basesintered body. The above high-melting metallizing layer is one formed onthe base through an oxide film made of SiO₂, a layer of one or moreGroup 4A metals or of a brazing material containing these, or through aCu--Cu₂ O eutectic layer, and has a peel strength of 3 kgf/mm² orhigher.

As described above, it is important in the conventional methods to use ahigh-purity Si₃ N₄ powder reduced in the contents of oxygen and cationicimpurities and to add an appropriate kind of sintering aid in anappropriate amount in order to inhibit oxygen ions and cationicimpurities from forming a solid solution in crystal grains. Namely, inorder to obtain crystal grains in which impurities or defects have beendiminished, the purity of the grains should be increased by usingexpensive high-purity powder feedstocks as the main and minoringredients and causing grain growth at a high temperature and a highpressure. For example, as described in Japanese Patent Laid-Open No.30866/1997, cited above, it is necessary to employ a method in whichhigh-purity β-form silicon nitride is used as a feedstock powder andgrain growth is caused at a high temperature and a high pressure (2,000°C., 300 atm).

DISCLOSURE OF INVENTION

As described above, improvements for obtaining a silicon nitride basesintered body having high strength and high thermal conductivity havehitherto been made by properly controlling a feedstock powder and asintering aid. However, merely selecting a proper feedstock powder and aproper sintering aid not only results in increased feedstock and processcosts but also is limited in further improving the thermal conductivityof a silicon nitride base sintered body.

There is hence a desire for another means for further reducing theamount of impurities, especially the amount of oxygen, contained in Si₃N₄ crystal grains. However, it is not easy to inexpensively obtain asilicon nitride powder reduced in oxygen content. In addition, sincegeneral silicon nitride powders on the market have an oxygen content ofat least 0.7 to 1.0% by weight, the thermal conductivities of thesilicon nitride base sintered bodies obtained from such inexpensivecommercial powder feedstocks have been limited to about 70 W/m·k.

Even when a high-purity β-form silicon nitride powder having arelatively low oxygen content is used, high-temperature high-pressuresintering is necessary for obtaining high thermal conductivity. Thismethod is therefore inferior in productivity because of exceedingly highfeedstock and production costs, etc., and is disadvantageous in that thesintered body is apt to have a reduced strength and poor suitability forpractical use since the treatment conducted at a temperature as high asaround 2,000° C. is accompanied with considerable grain growth.

In view of such prior art circumstances, an object of this invention isto provide a silicon nitride base sintered body having excellentproductivity and high strength and simultaneously having high thermalconductivity not possessed by any conventional silicon nitride basesintered body and to provide a process for producing the sintered body.

The silicon nitride base sintered body provided by this invention inorder to accomplish the above object is characterized by comprising aphase comprising crystal grains of β-form silicon nitride and a grainboundary phase containing a compound of at least one element selectedfrom the group consisting of yttrium and the lanthanide elements in anamount of 1 to 20% by weight in terms of oxide amount, and by containingfree silicon dispersed in the crystal grains of silicon nitride in anamount of 0.01 to 10% by weight based on the whole. The sintered bodycombines high strength and high thermal conductivity. As used hereinthroughout the specification and claims, the expression "lanthanideelements" is intended to include elements of atomic numbers 57 through71.

The high thermal conductive silicon nitride base sintered body of thisinvention can contain a compound of at least one element selected amongthe Group 4A elements in an amount of 0.01 to 3% by weight in terms ofelement amount and/or contain a compound of at least one elementselected from the group consisting of calcium and lithium in an amountof 0.1 to 5% by weight in terms of oxide amount. In this high thermalconductive silicon nitride base sintered body, the amount of oxygencontained in the crystal grains of silicon nitride is preferably 0.6% byweight or smaller.

The process for producing the above-described high thermal conductivesilicon nitride base sintered body of this invention is characterized bycomprising: a mixing step in which a silicon powder in an amount of 99to 80% by weight in terms of Si₃ N₄ is mixed with 1 to 20% by weightpowder of a compound of at least one element selected from the groupconsisting of yttrium and the lanthanide elements; a molding step inwhich the powder mixture is molded; a nitriding step in which theresultant compact is heated in an atmosphere containing nitrogen at1,200 to 1,400° C. to nitride the same until the amount of free siliconis reduced to 0.01 to 10% by weight based on the whole; and a sinteringstep in which the nitrided body is sintered by heating in an atmospherecontaining nitrogen at 1,600 to 2,000° C.

In the nitriding step in the process of this invention, the compact ispreferably heated at a rate of 0.3 to 0.5° C./min in the temperaturerange of from 1,200 to 1,300° C. and then heated in the temperaturerange of from 1,300 to 1,400° C. In the mixing step, it is preferred toadd a powder of a compound of at least one element selected among theGroup 4A elements or use a feedstock powder containing the Group 4Aelement so that the amount of the Group 4A element is 0.01 to 3% byweight based on the whole. Furthermore, a powder of a compound of atleast one element selected between lithium and calcium can be added inan amount of 1 to 5% by weight in terms of oxide amount based on thewhole.

BEST MODE FOR CARRYING OUT THE INVENTION

In this invention, it has become possible to obtain defect-freehigh-purity Si₃ N₄ crystal grains by a new method in which a siliconpowder which can be easily available with a high-purity is used as themain feedstock powder and nitrided into Si₃ N₄ while leaving an adequateamount of free silicon. Hence, an inexpensive silicon nitride basesintered body combining high thermal conductivity with mechanicalstrength can be obtained.

Namely, in the process of this invention, a silicon feedstock powder ismixed with 1 to 20% by weight compound of at least one rare earthelement selected from the group consisting of yttrium and the lanthanideelements, and a compact of the resultant mixture is nitrided at 1,200 to1,400° C. to obtain a nitrided body comprising high-purity Si₃ N₄crystal grains containing 0.05 to 10% by weight free silicon.Thereafter, this nitrided body is sintered at 1,600 to 2,000 C. tothereby obtain a silicon nitride base sintered body having high strengthand high thermal conductivity.

Compared especially to the case where nitrided bodies are produced fromSi₃ N₄, powders obtained through a pulverization step, such ascommercial ready-made nitrided Si₃ N₄ powders, the process of thisinvention can yield a nitrided body comprising Si₃ N₄ crystal grainsreduced in defects such as dislocations. Furthermore, due to thenitriding method in which free silicon is left in the nitrided body, anincrease in the purity. of the Si₃ N₄ crystal grains (diminution ofoxygen and defects) in the subsequent sintering step can be easilyattained. As a result, a silicon nitride base sintered body having agreatly improved thermal conductivity and a high strength is obtained.

The high thermal conductive silicon nitride base sintered body of thisinvention is constituted substantially of β-form silicon nitride and0.01 to 10% by weight free silicon finely dispersed in crystal grains ofthe silicon nitride, and contains as a grain boundary phase at least onerare earth element in an amount of 1 to 20% by weight in terms of oxideamount.

The amount of the free silicon dispersed in the silicon nitride crystalgrains is 0.01 to 10% by weight, preferably 0.01 to 5% by weight, basedon the whole sintered body. The dispersed silicon particles aredesirably fine particles specifically having a maximum diameter of 3 μmor smaller. If the amount of the dispersed silicon particles is below0.01% by weight, the sintered body has reduced thermal conductivity. Ifthe amount thereof exceeds 10% by weight, the sintered body has reducedflexural strength and reduced heat resistance. The reason why thesilicon nitride grains should be β-form is that the β-form is reduced incrystal strain and in phonon scattering as compared with the α-form andhence has excellent thermal conductivity.

The grain boundary phase contains a compound of at least one rare earthelement selected from the group consisting of yttrium and the lanthanideelements, and its content is 1 to 20% by weight in terms of oxide amountbased on the whole sintered body. If the content of the compound isbelow 1% by weight, the nitriding reaction proceeds insufficiently,making it difficult to regulate the amount of free silicon to a targetvalue. If the content thereof exceeds 20% by weight, a liquid phase ispresent in excess during sintering, resulting in a sintered body reducedin both thermal conductivity and flexural strength. Either case is henceundesirable. Especially preferred rare earth elements are those havingan ionic field strength [(valence/(ionic radius)² ] of 0.54 or higher,e.g., samarium, yttrium, ytterbium, gadolinium, dysprosium, and erbium.

The grain boundary phase can contain a compound of at least one elementselected among the Group 4A elements, besides the rare earth elementcompound. Due to the addition of at least one Group 4A element compound,the thermal conductivity of the sintered body can be further improved.This is because the Group 4A element compound is thought to function toenable the yielded Si₃ N₄ crystal grains to be considerably reduced inimpurity amount and in the amount of crystal strain attributable toimpurities. The content of Group 4A element compounds is preferably 0.01to 3% by weight in terms of element amount based on the whole. If thecontent thereof is below 0.01% by weight, the effect of furtherheightening thermal conductivity cannot be obtained. If the contentthereof exceeds 3% by weight, there are cases where a mechanicalstrength sufficient for practical use cannot be obtained.

The grain boundary phase may contain at least one element selected fromthe group consisting of calcium and lithium in an amount of 0.1 to 5% byweight in terms of its oxide. These elements improve suitability forsintering and contribute to densification in low-temperature sintering.The reason for the above amount range is that calcium or lithiumcontents below 0.1% by weight are ineffective in improving suitabilityfor sintering, while contents thereof exceeding 5% by weight result in asintered body reduced in mechanical strength.

The amount of oxygen contained in the Si₃ N₄ crystal grains in thesilicon nitride base sintered body is desirably 0.6% by weight orsmaller, preferably 0.3% by weight or smaller. By reducing the oxygenamount to such a low value, even higher thermal conductivity can bestably obtained.

The process for producing a silicon nitride base sintered body of thisinvention is explained next. First, in the mixing step in the process ofthis invention, a weighed amount of a silicon powder as the mainingredient is mixed with a weighed amount of a powder of a compound ofat least one rare earth element selected from the group consisting ofyttrium and the lanthanide elements as a minor ingredient. The amount ofthe silicon powder is 99 to 80% by weight in terms of Si₃ N₄, while thatof the minor ingredient powder is 1 to 20% by weight in terms of oxideamount. The mixing may be conducted by a known method.

The silicon powder for use as the main ingredient has anintraparticulate oxygen content(oxygen content in silicon particles) ofdesirably 0.6% by weight or lower, preferably 0.3% by weight or lower.This is because if a silicon powder having an intraparticulate oxygencontent exceeding 0.6% by weight is used, the oxygen content in the Si₃N₄ crystal grains obtained through the later nitriding step increasesand such an increased oxygen content becomes an obstacle to higherthermal conductivity. The average particle diameter of the siliconpowder is desirably 20 μm or smaller, preferably 5 μm or smaller. Thisis because if the average particle diameter thereof exceeds 20 μm, thereis a possibility that nitriding might proceed insufficiently in thenitriding step.

The kind and proportion of the minor ingredients which respectively arewithin the ranges specified above are intended to enable the target Si₃N₄ base sintered body to have a structure containing free silicon finelydispersed therein in an amount of 0.01 to 10% by weight and to therebyhave an improved strength and improved thermal conductivity. Inparticular, when a compound of an element having a high ionic fieldstrength (0.54 or higher), e.g., yttrium, samarium, or ytterbium, isadded, the element combines with free oxygen ions in the SiO₂ filmpresent on the silicon powder surface to inhibit the oxygen from forminga solid solution in the Si₃ N₄ crystal grains. The addition of such acompound is hence preferred for enhancing thermal conductivity. If thetotal amount of the minor ingredient is below 1% by weight, thenitriding reaction does not proceed sufficiently and unnitrided siliconremains in excess. The resultant silicon agglomerates serve as sitesfrom which breakage occurs, resulting in considerably reduced strengthproperties. If the amount thereof exceeds 10% by weight, a grainboundary phase is formed in an excess amount, resulting in a reducedstrength and reduced thermal conductivity.

Besides these powders of the main and minor ingredients, a compound ofat least one Group 4A element, e.g., titanium, zirconium, or hafnium,may be optionally incorporated in an amount of 0.01 to 3% by weight interms of element amount based on the whole, by adding a powder of thecompound or using a feedstock containing the compound as an impurity.These Group 4A elements are effective in improving thermal conductivityas long as they are used in an amount within the above range.

It is also possible to add a powder of a compound of lithium and/orcalcium, especially of the oxide(s), in an amount of 0.1 to 5% by weightbased on the whole. This addition can improve the sintering propertieswithout influencing thermal conductivity, and is especially effective instrength enhancement through low-temperature sintering. Lithium does notform a solid solution in Si₃ N₄ crystal grains because it volatilizesduring sintering, while calcium also is less apt to form a solidsolution in the crystal grains because it has a large ionic radius.Consequently, the sintered body can retain the excellent thermalconductivity.

In the molding step as the second step, the mixed feedstock powderobtained is molded to obtain a compact in a given shape. An ordinarymolding method can be used, such as the generally used mold pressingmethod or sheet forming method.

The nitriding step as the third step in this invention is conducted in anitrogen atmosphere at 1,200 to 1,400° C. Nitriding temperatures below1,200° C. are undesirable in that the reaction rate is considerably low,resulting in a sintered body having reduced mechanical properties. Incontrast, nitriding temperatures exceeding 1,400° C. are undesirable inthat since the compact is partly heated to or above the melting point ofsilicon, silicon melting occurs and the melted silicon remains as coarseunnitrided agglomerates, resulting in a sintered body having reducedmechanical properties.

In this nitriding step, the compact is preferably heated at a rate of0.3 to 0.5° C./min in the temperature range of especially from 1,200 to1,300° C. and then heat-treated in the temperature range of from 1,300to 1,400° C. This is because thus controlling the heating rate issuitable for regulating the free silicon remaining unnitrided so as tobe present in a desirable dispersed state and in a desirable amount. Iftoo high a heating rate is used, free silicon forms coarse agglomeratesdue to the heat generated by the reaction and is hence less apt to comeinto the desired, evenly and finely dispersed state. If the heating rateis lower than the lower limit, impurities are apt to form a solidsolution in the Si₃ N₄ crystal grains being yielded.

In the sintering step as the final step, the nitrided compact which hasundergone the nitriding step is sintered in a nitrogen atmosphere at1,600 to 2,000° C. If the sintering temperature is below 1,600° C., theresultant sintered body has an increased porosity and hence a reducedthermal conductivity. Conversely, sintering temperatures exceeding2,000° C. are undesirable in that the Si₃ N₄ is apt to decompose.Especially in the case where sintering is conducted at ordinarypressure, a temperature of 1,800° C. or lower is desirably used. It isalso preferred to place the nitrided body in a vessel made of carbon orto use a furnace whose inner wall is made of carbon, in order to preventexternal oxygen inclusion. The nitrogen atmospheres for use in thenitriding step and sintering step may contain ammonia gas or other inertgases, besides nitrogen.

In this sintering step, the Si₃ N₄ grains in the nitrided body changefrom the α-form to the β-form to thereby form a network structurecomprising densified columnar crystal grains. In the process of thisinvention, a dense, high thermal conductive silicon nitride basesintered body can be obtained usually through sintering at 1,700 to1,900° C. in a nitrogen atmosphere having a pressure of about 1 to 5atm. Unlike conventional sintering steps, the sintering in this processneed not be conducted, for example, at a temperature as high as about2,000° C. and a pressure as high as 100 atm or higher.

The silicon nitride base sintered body obtained by the above-describedprocess of this invention has high thermal conductivity together withexcellent mechanical strength. Specifically, a silicon nitride basesintered body having a relative density of 95% or higher, a thermalconductivity of 50 W/m·k or higher, and a flexural strength of 600 MPaor higher can be provided by the inexpensive production process.

EXAMPLES Example 1

Various silicon powders having the intraparticulate oxygen contents andaverage particle diameters shown in the following Table 1 and an Sm₂ O₃power having an average particle diameter of 0.5 μm were prepared. Eachsilicon powder and the Sm₂ O₃ powder were weighed out in such respectiveamounts that the silicon powder amount in terms of Si₃ N₄ is shown inTable 1 and the Sm₂ O₃ powder accounted for the remainder. The twopowders were mixed with each other in ethyl alcohol by means of a ballmill. The resultant slurry was dried and granulated with a spray dryerto obtain a granular powder mixture having an average particle diameterof about 100 μm.

                  TABLE 1                                                         ______________________________________                                               Silicon powder    Amount                                                        Particle Oxygen           Of Sm.sub.2 O.sub.3                                 diameter content    Amount                                                                              Powder                                     Sample   (μm)  (wt %)     (wt %)                                                                              (wt %)                                     ______________________________________                                         1*      0.05      1.0       90    10                                         2        1         0.6       90    10                                         3        5         0.4       90    10                                         4        10        0.3       90    10                                         5        20        0.3       90    10                                         6        25        0.3       90    10                                          7*      25        1.0       90    10                                          8*      5         0.4       78    22                                         9        5         0.4       60    20                                         10       5         0.4       90    10                                         11       5         0.4       99    1                                          12*      5         0.4       99.5  0.5                                        ______________________________________                                         (Note)                                                                        The asterisked samples in the table are comparative examples.            

Each granular powder mixture was molded by dry pressing into test piecesin two forms, i.e., test pieces having a length of 45 mm, a width of 8mm, and a thickness of 5 mm (for flexural strength measurement) and oneshaving an diameter of 12.5 mm and a thickness of 5 mm (for thermalconductivity measurement). Thereafter, these compacts were placed in arefractory case made of carbon and lined with BN, nitrided in 1-atmnitrogen gas at 1,300° C. for 3 hours, and successively heated to 1,850°C. to conduct 3-hour sintering in 4-atom nitrogen gas.

As a result of X-ray diffractometry, all Si₃ N₄ crystal grains in eachsintered body obtained were ascertained to be β-form. The two kinds oftest pieces for each sintered body were examined for relative density(proportion of the found density value measured by the Archimedes methodto the theoretical density) and for three-point flexural strength andthermal conductivity (laser flash method). Furthermore, the amount ofoxygen contained in the Si₃ N₄ crystal grains was ascertained by Augerelectron spectroscopy or the XPS method. With respect to the nitridedcompacts which had undergone the nitriding step and the sintered bodiesobtained after the sintering step, the amount of free silicon wasascertained by EPMA or Auger electron spectroscopy. In each sample, themaximum diameter of the silicon particles was about 0.8 μm. The resultsof the above examinations are shown in the following Table 2.

                  TABLE 2                                                         ______________________________________                                        Silicon                                                                       amount    Si.sub.3 N.sub.4 sintered body                                            in                     Thermal                                                nitrided        Flexural                                                                             Conduc-                                                                              Silicon                                                                              Oxygen                                   body    Density strength                                                                             tivity Amount amount                             Sample                                                                              (wt %)  (%)     (MPa)  (W/mK) (wt %) (wt %)                             ______________________________________                                         1*   <0.01   99      1000   40     <0.01  1.0                                2     0.5     99      1000   85     0.5    0.4                                3     2.0     99      900    90     2.0    0.3                                4     4.0     97      800    92     4.0    0.2                                5     7.0     90      700    80     8.0    0.3                                6     8.0     89      500    70     8.0    0.3                                 7*   15      80      400    40     15     1.0                                 8*   <0.01   80      500    45     <0.01  0.7                                9     0.1     99      850    80     0.1    0.3                                10    1.0     99      900    90     1.0    0.3                                11    10      80      600    70     10     0.4                                12*   20      70      400    40     20     0.8                                ______________________________________                                         (Note)                                                                        The asterisked samples in the table are comparative examples.            

The above results show the following. The content of silicon in β-formSi₃ N₄ crystal grains obtained after sintering can be regulated withinthe range of 0.01 to 10% by weight and a high thermal conductive Si₃ N₄base sintered body having a three-point flexural strength of 600 MPa orhigher and a thermal conductivity of 50 W/m·k or higher is obtained byusing a silicon feedstock power having an oxygen content of 1% by weightor lower adding a rare earth oxide (Sm₂ O₃) powder as a minor ingredientin an amount of 1 to 20% by weight, and nitriding the mixture of both ina nitrogen atmosphere at 1,200 to 1,400° C.

Example 2

The same feedstock powders as those used for sample 3 in Example 1 wereused in the same proportion to likewise prepare compacts in the givenshapes. The compacts were nitrided for 3 hours in 1-atm nitrogen gas ateach of the nitriding temperatures shown in the following Table 3. Inthis nitriding, some of the samples were regulated with respect toheating rate in the temperature range of from 1,200 to 1,300° C. asshown in Table 3. Thereafter, the nitrided bodies were sintered for 3hours in 4-atm nitrogen gas at each of the sintering temperatures shownin Table 3. The Si₃ N₄ base sintered body samples obtained wereevaluated in the same manner as in Example 1. The results of theevaluations are shown in the following Table 4.

                  TABLE 3                                                         ______________________________________                                                             Silicon                                                         Nitriding Conditions                                                                        amount in                                                         Heating  Nitriding  nitrided                                                                             Sintering                                          rate     Treatment  body   conditions                                Sample   (° C./min)                                                                      (° C. xhr)                                                                        (wt %) (° C. xhr)                         ______________________________________                                        13       0.3      1300 × 3                                                                           0.5    1850 × 3                            14       0.5      1300 × 3                                                                           1.2    1850 × 3                            15       0.7      1300 × 3                                                                           1.5    1850 × 3                             16*     --       1180 × 3                                                                           30     1850 × 3                             17*     --       1200 × 3                                                                           20     1850 × 3                            18       0.4      1300 × 3                                                                           1.0    1850 × 3                            19       0.4      1400 × 3                                                                           0.3    1850 × 3                             20*     0.4      1420 × 3                                                                           5.0    1850 × 3                             21*     0.4      1300 × 3                                                                           1.0    1580 × 3                            22       0.4      1300 × 3                                                                           1.0    1600 × 3                            23       0.4      1300 × 3                                                                           1.0    1700 × 3                            24       0.4      1300 × 3                                                                           1.0    1800 × 3                            25       0.4      1300 × 3                                                                           1.0    1900 × 3                            26       0.4      1300 × 3                                                                           1.0    2000 × 3                             27*     0.4      1300 × 3                                                                           1.0    2200 × 3                            ______________________________________                                         (Note)                                                                        The asterisked samples in the table are comparative examples.            

                  TABLE 4                                                         ______________________________________                                        Si.sub.3 N.sub.4 sintered body                                                                        Thermal                                                               Flexural                                                                              conduc-  Silicon                                                                             Oxygen                                       Density   Strength                                                                              tivity   amount                                                                              amount                                 Sample                                                                              (%)       (MPa)   (W/mK)   (wt %)                                                                              (wt %)                                 ______________________________________                                        13    99        950     110      0.5   0.2                                    14    99        900     110      1.2   0.2                                    15    99        900     92       1.5   0.3                                     16*  75        350     50       20    0.7                                     17*  80        500     55       10    0.7                                    18    99        1000    120      1.0   0.15                                   19    99        1100    110      0.3   0.2                                     20*  90        400     70       5     0.6                                     21*  70        550     50       1.0   0.3                                    22    90        850     90       1.0   0.2                                    23    98        900     100      1.0   0.2                                    24    99        1000    115      1.0   0.15                                   25    99        1100    115      1.0   0.15                                   26    95        900     95       1.0   0.2                                     27*  85        500     80       1.0   0.4                                    ______________________________________                                         (Note)                                                                        The asterisked samples in the table are comparative examples.            

The above results show that by using a nitriding temperature of 1,200 to1,400° C., the Si₃ N₄ crystal grains in the resultant nitrided bodyinclude therein silicon in a content range of from 0.01 to 10% byweight. The results further show that by sintering the nitrided body at1,600 to 2,000° C., a high thermal conductive Si₃ N₄ base sintered bodycan be obtained in which the silicon content in the Si₃ N₄ crystalgrains is in the range of 0.01 to 10% by weight and which has athree-point flexural strength of 600 MPa or higher and a thermalconductivity of 50 W/m·k or higher. The results furthermore show that byregulating the heating rate in the temperature range of from 1,200 to1,300° C. during nitriding to 0.3 to 0.5° C./min, sintered bodies evenmore improved in three-point flexural strength and thermal conductivitycan be obtained.

Example 3

Powder mixtures were prepared using the same silicon powder and Sm₂ O₃powder as those for sample 4 in Example 1 in such a manner that thesilicon powder was used in an amount of 90% by weight and part of the10% by weight Sm₂ O₃ powder as the remainder was replaced with each ofthe substitute compound powders shown in the following Table 5. Usingeach of the powder mixtures, compacts were prepared in the same manneras Example 1. The compacts were nitrided under the same conditions asthose for sample 15 in Example 2 and then sintered for 3 hours in 5-atmnitrogen gas respectively at the sintering temperatures shown in Table5. The sintered bodies obtained were evaluated in the same manner as inExample 1. The results of the evaluations are shown in the followingTable 6.

                  TABLE 5                                                         ______________________________________                                        Substitute compound Nitrided body                                             Powder                  Silicon Sintering                                            Kind of the                                                                             Amount     Amount                                                                              Temperature                                 Sample compound  (wt %)     (wt %)                                                                              (° C.)                               ______________________________________                                        28     TiN       0.005      1.5   1800                                        29     TiN       0.01       1.5   1800                                        30     TiN       0.1        1.5   1800                                        31     TiN       1          2.0   1800                                        32     TiN       3          4.0   1800                                        33     TiN       4          12.0  1800                                        34     ZrC       1          2.0   1800                                        35     CaO       0.08       1.5   1800                                        36     CaO       0.1        1.5   1750                                        37     CaO       2          1.5   1650                                        38     CaO       5          2.0   1620                                        39     CaO       6          10.0  1600                                        40     Li.sub.2 O                                                                              0.08       1.5   1750                                        41     Li.sub.2 O                                                                              0.1        1.5   1700                                        42     Li.sub.2 O                                                                              2          1.5   1650                                        43     Li.sub.2 O                                                                              6          6.0   1630                                        ______________________________________                                         (Note)                                                                        The asterisked samples in the table are comparative examples.            

                  TABLE 6                                                         ______________________________________                                        Si.sub.3 N.sub.4 sintered body                                                                        Thermal                                                               Flexural                                                                              conduc-  Silicon                                                                             Oxygen                                       Density   Strength                                                                              tivity   amount                                                                              amount                                 Sample                                                                              (%)       (MPa)   (W/mK)   (wt %)                                                                              (wt %)                                 ______________________________________                                        28    99        900     92       1.5   0.2                                    29    99        900     105      1.5   0.15                                   30    99        900     110      1.5   0.15                                   31    99        900     120      2.0   0.15                                   32    98        800     105      4.0   0.15                                   33    90        500     90       10.0  0.2                                    34    99        800     105      2.0   0.2                                    35    99        900     90       1.5   0.2                                    36    99        1100    90       1.5   0.2                                    37    99        1000    90       1.5   0.2                                    38    98        900     85       2.0   0.3                                    39    85        600     60       8.0   0.6                                    40    99        800     90       1.5   0.3                                    41    99        950     90       1.5   0.3                                    42    99        920     90       1.5.  0.3                                    43    80        450     60       6.0   0.3                                    ______________________________________                                    

The above results show that the addition of a Group 4A element compoundin an amount of 0.01 to 3% by weight in terms of element amount iseffective in obtaining a sintered body having an even more improvedthermal conductivity while retaining the high flexural strength. Theresults further show that the addition of a calcium or lithium compoundin an amount of 0.1 to 5% weight in terms of oxide amount is effectivein improving the sinterability and in thus enabling low-temperaturedensification. The results furthermore show that although the additionof these ingredients is effective, it leads to a decrease in thestrength of a sintered body when the amount of the Group 4A elementexceeds 3% by weight or that of the calcium or lithium compound exceeds5% by weight.

Example 4

A silicon powder having an average particle diameter of 1 μm and anintraparticulate oxygen content of 0.5% by weight and the rare earthelement compound powders described in the following Table 7 which eachhad an average particle diameter of 0.5 μm were prepared. The siliconpowder and each rare earth element compound powder were weighed out sothat the silicon powder amount in terms of Si₃ N₄ was 90% by weight andthe other powder accounted for the remainder, i.e., 10% by weight.Powder mixtures were prepared in the same manner as in Example 1.Thereafter, each powder mixture was molded into compacts in the twoforms in the same manner as in Example 1. The resultant shapes wereplaced in a refractory case made of carbon and lined with Si₃ N₄, andthen nitrided and sintered under the same conditions as in Example 1.

X-ray diffractometry revealed that all the Si₃ N₄ crystal grains in eachof the sintered bodies obtained were β-form. The sintered bodies eachhad a relative density of 98 to 99%. Each sintered body was evaluatedfor silicon amount and oxygen amount in the crystal grains and forthree-point flexural strength and thermal conductivity by the samemethods as in Example 1. The results of the evaluations are shown inTable 7. The results show that the addition of a rare earth elementhaving a field strength [valence/(ionic radius)² ] of 0.54 or higher iseffective in obtaining an even higher thermal conductivity.

                  TABLE 7                                                         ______________________________________                                        Rare earth compound                                                                           Si.sub.3 N.sub.4 sintered body                                powder                  Thermal                                                    Kind of  Field     Flexural                                                                            Conduc-                                                                              Silicon                                                                             Oxygen                             Sam- the Com- strength of                                                                             strength                                                                            tivity amount                                                                              amount                             ple  pound    the element                                                                             (MPa) (W/mk) (wt %)                                                                              (wt %)                             ______________________________________                                        44   La.sub.2 O.sub.3                                                                       0.507     700   60     1.8   0.6                                45   CeO.sub.2                                                                              0.517     750   65     2.2   0.6                                46   Nd.sub.2 O.sub.3                                                                       0.528     800   60     2.1   0.6                                47   Sm.sub.2 O.sub.3                                                                       0.540     1000  90     0.5   0.4                                48   Y.sub.2 O.sub.3                                                                        0.567     800   85     2.0   0.3                                49   Yb.sub.2 O.sub.3                                                                       0.583     1000  105    1.0   0.2                                ______________________________________                                    

Example 5

Silicon powders respectively having the intraparticulate oxygen contentsshown in the following Table 8 and powders of the rare earth elementcompounds shown in Table 8 which each had an average particle diameterof 0.5 μm were prepared. Each silicon powder each rare earth elementcompound powder were weighed out so that the silicon powder amount interms of Si₃ N₄ was 90% by weight and the other powder accounted for theremainder, i.e., 10% by weight. Granular powder mixtures were preparedin the same manner as in Example 1. Thereafter, each granular powdermixture was nitrided and sintered under the same conditions as those forsample 18 in Example 2. The sintered bodies obtained were evaluated inthe same manner as in Example 1. The results of the evaluations areshown in Table 8. The results show that the lower the intraparticulateoxygen content of the starting silicon powder, the higher the thermalconductivity of the Si₃ N₄ sintered body obtained.

                  TABLE 8                                                         ______________________________________                                        Oxygen                                                                        content            Si.sub.3 N.sub.4 sintered body                                   of                     Thermal                                                Silicon Rare     Flexural                                                                            conduct                                                                              Silicon                                                                              Oxygen                                   powder  earth    strength                                                                            ivity  Amount amount                             Sample                                                                              (wt %)  compound (MPa) (W/mk) (wt %) (wt %)                             ______________________________________                                        50    0.2     La.sub.2 O.sub.3                                                                        700   90    1.5    0.3                                51    0.2     Sm.sub.2 O.sub.3                                                                        900  130    1.0    0.1                                52    0.3     Yb.sub.2 O.sub.3                                                                       1100  120    0.5    0.15                               53    0.1     Yb.sub.2 O.sub.3                                                                       1000  130    0.5    0.1                                ______________________________________                                    

INDUSTRIAL APPLICABILITY

According to this invention, a silicon nitride base sintered bodycombining high strength with high thermal conductivity not possessed byany conventional silicon nitride base sintered body can be provided by anovel process having excellent productivity by removing or diminishingimpurities which can form a solid solution in silicon nitride crystalgrains. This high thermal conductive silicon nitride base sintered bodyis extremely useful not only as various parts for semiconductor devices,such as radiating insulating substrates, but as various structural partsfor machines, OA apparatuses, etc.

What is claimed is:
 1. A high thermal conductive silicon nitride basesintered body which comprises a phase comprising crystal grains ofsilicon nitride and a grain boundary phase containing a compound of atleast one element selected from the group consisting of yttrium and thelanthanide elements in an amount of 1 to 20% by weight in terms of oxideamount, and contains free silicon dispersed therein in an amount of 0.01to 10% by weight based on the whole.
 2. A high thermal conductivesilicon nitride base sintered body as set forth in claim 1, wherein theamount of oxygen contained in the crystal grains of silicon nitride is0.6% by weight or smaller.
 3. A high thermal conductive silicon nitridebase sintered body as set forth in claim 1, which contains a compound ofat least one element selected among the Group 4A elements in an amountof 0.01 to 3% by weight in terms of element amount.
 4. A high thermalconductive silicon nitride base sintered body as set forth in claim 1,which contains a compound of at least either element selected from thegroup consisting of calcium and lithium in an amount of 0.1 to 5% byweight in terms of oxide amount.
 5. A high thermal conductive siliconnitride base sintered body as set forth in claim 1, which has a relativedensity of 95% or higher, a thermal conductivity of 50 W/m·k or higher,and a flexural strength of 600 MPa or higher.
 6. A process for producinga high thermal conductive silicon nitride base sintered body, whichcomprises: a mixing step in which a silicon powder in an amount of 99 to80% by weight in terms of Si₃ N₄ is mixed with 1 to 20% by weight powderof a compound of at least one element selected from the group consistingof yttrium and the lanthanide elements; a molding step in which thepowder mixture is molded; a nitriding step in which the resultantcompact is heated in an atmosphere containing nitrogen at 1,200 to1,400° C. to nitride the same until the amount of free silicon isreduced to 0.01 to 10% by weight based on the whole; and a sinteringstep in which the nitrided body is sintered by heating in an atmospherecontaining nitrogen at 1,600 to 2,000° C.
 7. A process for producing ahigh thermal conductive silicon nitride base sintered body as set forthin claim 6, wherein the silicon powder has a content of intraparticulateoxygen of 0.6% by weight or lower.
 8. A process for producing a highthermal conductive silicon nitride base sintered body as set forth inclaim 6, wherein in the nitriding step, the compact is heated at a rateof 0.3 to 0.5° C./min in the temperature range of from 1,200 to 1,300°C. and then heat-treated in the temperature range of from 1,300 to1,400° C.
 9. A process for producing a high thermal conductive siliconnitride base sintered body as set forth in claim 6, wherein in themixing step, a powder of a compound of at least one element selectedamong the Group 4A elements is added or a feedstock powder containingthe Group 4A element is used so that the amount of the Group 4A elementis 0.01 to 3% by weight based on the whole.
 10. A high thermalconductive silicon nitride base sintered body as set forth in claim 6,wherein in the mixing step, a powder of a compound of at least oneelement selected between lithium and calcium is added in an amount of 1to 5% by weight in terms of oxide amount based on the whole.